We have perfomed 19 studies under this protocol. All patients underwent invasive catheterization study, and a subset (n=11) underwent subsequent permanent dual chwnbff pacemaker implantation. Our data were recently presented at the American College of Cardiology Meeting in Atlanta. The major results an that left ventricular pacing (single site) can increase ventricular contractility by nearly 25% with minimal effects on chamber filling pressures or volumes. Cardiac stroke work and function are also improved as is cardiac output. Right heart pacing at either septal or apical sites (the latter being the standard approach used in >99% of pacemaker implants) do not significantly alter LV function in these patients. Biventricular pacing has been found to poduce results similar or worse than that with single site LV pacing alone. Our data are the first to demonstrate the mechanism of pacing-induced cardiac benefit in heart failure patients. There have been few complications. One patient developed a small chronic pericardial effusion, while another had a post-operative effusion that required drainage. Chronic data are more limited due to the recent implementation of left heart pacing, and the protocol is currently being amended so that chronic pacing can be instituted via the coronary sinus (coronary venous pacing) rather than by limited thoracotomy. The data from this initial 19 patients is currently being prepared for journal submission.